1. Field of the Invention
The present invention relates to a battery discharge characteristics calculation method and a remaining battery capacity measuring device, and more specifically, to a battery discharge characteristics calculation method with which the discharge characteristics can be determined only through sampling voltage, current, and temperature, and a remaining battery capacity measuring device capable of easily determining a remaining battery capacity by using the current discharge characteristics rather than using a table having two or more discharge characteristics.
2. Description of the Related Art
In general, it would be the best possible to determine the remaining capacity of a battery as the result of subtraction of a discharge amount obtained out of a load current and a terminal voltage from the fully charged amount. However, in general, batteries have discharge characteristics which are different depending upon the variation of load connected thereto, an ambient temperature, aged deterioration and the like. Therefore, the remaining capacity of batteries is different depending upon variation of a load, an ambient temperature, aged deterioration and the like.
For example, as shown in FIG. 1, even if the temperature is kept at a constant value of 20.degree. C., batteries have different discharge characteristics depending upon whether the current discharged from the battery is 10 A, 20 A, . . . or 80 A. When it is assumed that a usable voltage range is from 25 V to 20 V, the battery becomes unusable when it is operated for 160 minutes with a discharge current of 10 A and when it is operated for 40 minutes with a discharge current of 30 A. That is, batteries have a different discharge ratio and thus a different remaining capacity depending upon variation of the load.
In particular, since electric cars are powered by batteries, the remaining capacity of the battery must be accurately predicted. The following voltage and current detecting method is generally available to measure the remaining capacity of a battery.
This voltage and current detecting method uses a characteristics curve whose parameter is an index SOC (State of Charge) indicating the remaining battery capacity of each discharge characteristic from relationship between a load current and a terminal voltage of a battery shown in FIG. 1, and determines SOC from the measured voltage and current values by using these two or more characteristic curves. This method, however, requires the use of a database for storing data of a plurality of characteristic curves. For example, the device disclosed in Japanese Utility Model Registration Laid-open No. 4-115084 conducts the following operations.
FIG. 2 is a block diagram schematically showing a conventional remaining battery capacity meter for electric cars. As shown in FIG. 2, a voltage sensor, a current sensor, and a temperature sensor measure the terminal voltage of a battery for electric cars, a load current flowing through the battery, and the liquid temperature, respectively. These sensors produce signals indicative of the measured values. Subsequently, a signal amplifier amplifies the signals, an analog filter removes noise from the amplified signal, and an A/D converter converts the signal while the electric car is in operation.
Subsequently, moving average filters 2-1 to 2-3 are digital exchange filters for the smoothing operation. The remaining capacity calculation and processing unit 3 calculates the remaining capacity of the battery at a normal temperature of 30.degree. C. by using the temperature correction and I-V tables, and an interpolation formula. From the value of the remaining capacity, an average processing unit 4, a moving average processing unit 5, and an average processing unit 6 calculate the remaining capacity at the present liquid temperature again according to the conversion formula. The calculation result is smoothed again and then produced every 10 minutes on an LED display unit 7 as a graph and a value, indicating the remaining capacity of the battery.
Further, a velocity pulse counter 8 counts velocity pulses to determine a travel distance, and a travel distance calculation unit 9 receives a signal indicative of the remaining capacity from the average processing unit 6 to determine the distance to be further traveled from the determined travel distance and remaining capacity.
However, the voltage and current can abruptly change in accordance with a variation of the load. For example, in the case of electric cars, the remaining capacity of the battery is determined by means of summing regenerative currents generated when the electric car reduces speed. However, in the case of electric cars, the voltage and current abruptly change in accordance with a variation of the load.
Thus, unless the voltage, current, and their pick-up time are determined with high accuracy, the discharge characteristics cannot be accurately determined. This causes the problem of a complicated operation due to the needed of additional steps such as correcting to increase the accuracy.
Also, it would be the best possible to determine the remaining capacity of the battery as the result of subtraction of the discharge amount obtained out of the load current and terminal voltage from the fully charged amount, on the assumption that an electric car starts running from a fully charged state. In practice, however, the electric car may start not being fully charged. Even if it is fully charged, the electric car may have sat unused for a length of time.
More specifically, the discharge duration from a fully charged state to the current cannot be predicted. This means that calculating the remaining capacity from the fully charged amount can only be achieved with various discharge characteristics estimated out of a duration of discharge, temperature, and a detected voltage. Accordingly, the discharge characteristics for various cases should be stored previously in, for example, a database for the calculation, which brings with it problems of the need for increased memory capacity and more complicated operations.
Further, the device disclosed in Japanese Utility Model Registration Laid-open No. 4-115084 uses a method of picking up voltage, current, and temperature to estimate the remaining capacity. However, this method relies on the calculation of the remaining capacity of the battery at a normal temperature of 30.degree. C. by using the temperature correction and I-V tables, and an interpolation formula at the processing unit for estimating the remaining capacity at the current liquid temperature again according to the conversion formula. Thus, there exists a problem that an operation to determine the remaining capacity is complicated and the memory requirements and capacities are large.